Fire shutoff valve(Ball valve),Is there any guidelines available for selection of ball diameter in ball valve
And is there any relation available for orifice diameter to ball diameter.
You should be able to provide a bore hole diameter equal to pipe ID.
Draw a cube around the pipe and make the surface of the ball intersect the corners of the cube.  That's the minimum sphere diameter.
There wouldn't be much use for a larger ball than that.
For rule of thumb, the bore diameter is 60% of the sphere diameter.  That tends to be a minimum.  But you need to check the contact area with the seat.  The above ratio may not ensure full contact area all around the seat when the valve is shut.
The circumscribed cube method gives a full pipe diameter bore equal to 57.7% of sphere diameter.  We could call it 60%.

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